U.S. Pat. Re. No. 27,869 to Willits et al describes apparatus for counting stacked sheets having no sheet separation requirements. The active area of a sensor array is matched to the width of a sheet and the sensor array traverses the stack. The signal output of the sensor array is stripped of unwanted components in a high gain, diode clamped capacitive input operation amplifier whose square wave output is processed and counted by a counting circuit.
U.S. Pat. No. 5,005,192 to Duss describes a system for counting flat objects in a stream of partially overlapping objects which are conveyed past a locus of impingement of ultrasonic waves.
U.S. Pat. No. 4,694,474 to Dorman et al describes a device for counting a stack of thin objects in which light is directed at the stack and a light sensor generates a signal proportional to the light reflected by the stack.
U.S. Pat. No. 5,040,196 to Woodward describes an instrument for counting stacked elements which images a portion of the side of the stack and then autocorrelates the image, while the instrument is stationary, and then cross-correlates the image as the instrument is moved. The result is a time varying signal whose repeating cycles, when counted, indicate the number of elements in the stack.
U.S. Pat. No. 3,971,918 to Saito counts stacked corrugated cardboards by scanning an end of the stack horizontally and vertically, using an array of photodiodes switched in turn by electric pulses. The outputs of the photodiodes are counted and compared to successively detect flat and corrugated sheets.
U.S. Pat. No. 4,912,317 to Mohan et al describes apparatus for counting stacked sheets whose apparent brightness is not uniform. The Mohan et al system normalizes the phase polarity of the sensor signal differential output, thereby avoiding the effects of brightness polarity reversals in the sensor output data. Mohan et al employs sensors whose effective imaged width on the stacked objects is very narrow relative to the individual objects. The data is differentially summed, then rectified to normalize phase polarity.
None of the above U.S. Patents teaches that the devices described therein are suitable for counting banknotes.
U.S. Pat. No. 5,324,921 describes a conventional sheet counting machine in which a photosensor is disposed across a bill passage downstream of a pulley. Emitted light is interrupted by each bill passing throught the light path and therefore the number of bills can be counted by counting the number of intervals during which light is not received by the light receiver.
A general text on image processing is Pratt, W. K, Digital image processing, Second Ed., Wiley 1991, New York.
The disclosures of all of the above publications and of the references cited therein are hereby incorporated by reference.
Brandt, Inc. of Bensalem, Pa. 19020, USA, markets a Model 8640D Note Counter accomodating notes of at least a minimum note size and thickness and no more than a maximum note size and thickness. The 8640D leafs through the banknotes in order to determine the number of banknotes.